Microflora of the middle Proterozoic Balbirini Dolomite (McArthur Group) of Australia

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Abstract
A diverse assemblage of organic microfossils is preserved within stromatolitic cherts from 10 localities in the approximately 1500 m.y. old Balbirini Dolomite. The assemblage is dominated by coccoid blue-green algae referable to the Entophysalidaceae and the Chroococcaceae. There are a few oscillatoriacean and dermocarpacean or pleurocapsacean cyanophytes, and sporangia-like spheroids of uncertain affinities are abundant. A total of 14 genera including at least 25 species have been identified; however, at any particular locality, the species diversity is relatively low. Eoentophysalis belcherensis Hofmann is one of the most abundant organisms in the assemblage and appears to have formed the basically flat laminae of the stromatolites studied. These stromatolites seem analogous to modern examples of ‘pustular mat.’ Numerous degradational forms of E. belcherensis have been recognized. Cell-like units in this alga (probably remnants of sheaths) commonly are defined by opaque, possibly pyritic granules. These granules may have formed at sites in the sheaths where iron was chelated to a scytonemine-like pigment. Pyrite inclusions located within cells of Sphaerophycus reticulatum Muir appear to have formed at sites of coalesced organic matter. The algal mats in the Balbirini Dolomite grew in an arid, intertidal to supratidal habitat. Comparisons are made between the communities preserved at each locality and modern assemblages from different environmental zones in Hamelin Pool, Shark Bay. These comparisons are pertinent to palaeoenvironmental basin analyses. There is no firm evidence for the presence of eukaryotes in this biota, although 5 genera are of unclear affinities and could include prokaryotic and/or eukaryotic organisms. A fairly advanced state of the cyanophycean colonial habit is indicated by the organizational complexity exhibited by E. belcherensis, Myxomorpha janecekii Muir, and Myriasporella pyriformis gen. et sp. nov. The presence of numerous sporangia-like spheroids suggests that sporangial reproduction may have developed by middle Proterozoic time. E. belcherensis probably had a cosmopolitan distribution at least 1500 m.y. ago. The Balbirini microflora is comparable to those from the Amelia Dolomite (McArthur Group) and the approximately 2000 m.y. old Belcher Supergroup of Canada. It is likely that the predominance of Chroococcaceae and mat-forming Entophysalidaceae in all three assemblages reflects their similar depositional environments. Results of this study are applicable to the establishment of a biostratigraphic zonation of the McArthur Group and are consistent with the concept that the size distribution of unicells in Proterozoic microbiotas may be useful for intercontinental stratigraphic correlation. New taxa include 4 genera (Tetraphycus, Myriasporella, Balbiriniella, Pilavia) and 12 species (Chroococcaceae: Myxococcoides cracens; Chroococcaceae(?): Tetraphycus acinulus, T. diminutivus, T. gregalis, T. major; Dermocarpaceae or Pleurocapsaceae: Myriasporella pyriformis; Incertae Sedis: Balbiriniella praestans, Clonophycus biattina, C. ostiolum, C. refringens, C. vulgaris, Pilavia maculata).